Motor vehicles are increasingly equipped with so-called ACC (adaptive cruise control) systems which allow the distance between the host vehicle and a preceding vehicle to be automatically controlled. For this purpose, the distances and azimuths as well as relative velocities of preceding vehicles are measured with the aid of the radar sensor, e.g., an FMCW LRR (frequency-modulated continuous wave long-range radar) sensor. These radar sensors typically work with a frequency of 24 GHz or 77 GHz.
Lens antennas are, for example, used in the case of long-range radar (LRR) sensors for motor vehicles. These include a radar lens and multiple antenna elements which are also referred to as primary radiators. The primary radiators are, for example, configured as patch antennas. To obtain an angular misalignment of the primary sensitivity directions of the patch antennas, the primary radiators are situated in a series transversely to the optical axis of the radar sensor. If, as is often the case, the same antenna elements are used to emit the radar signal and to receive the radar echo, the primary radiation directions of the antenna elements also have corresponding deviations. The greater the distance a primary radiator is from the optical axis of the radar lens, the stronger does the primary radiation direction of the antenna element in question deviate from the optical axis. This deviation is also referred to as squinting. When specifying the widths of the sensitivity ranges of the antenna elements, a compromise must be made between a wide range of vision and a good angle resolution.
The radar sensors used so far in this context have four antenna elements or antenna patches, for example. Each antenna element is assigned exactly one channel of an evaluation device of the radar sensor. The antenna elements each have a directional characteristic having a main lobe, i.e., a limited range of high sensitivity which includes a sensitivity maximum. The main lobes of the antenna elements together cover a certain angle range. Since the sensitivity ranges overlap, radar echoes from a single radar object are received in multiple antenna elements and thus in multiple channels. For an idealized, approximately punctiform radar object at a given azimuth, there is a characteristic phase-amplitude relation between the signals received in the different channels. Due to the propagation differences of the radar echoes from the radar object to the different antenna elements, a phase difference results which is approximately proportional to the azimuth with regard to an optical axis of the radar sensor and proportional to the distance between the antenna elements in the direction which is at a right angle to the optical axis, as well as inversely proportional to the wave length of the radar waves. The amplitude ratios between the received signals are a function of the azimuth and of the sensitivity curves of the antenna elements. By evaluating the phase relations and/or by evaluating the amplitude relations, it is possible to determine the azimuth of a located radar object.
The signal received by an assigned antenna element is evaluated in the channels of the radar sensor. In an FMCW radar, for example, in which the frequency of the transmitted radar signal is modulated periodically, the received signal is mixed for each antenna element with the signal transmitted at the receiving point in time, by maintaining the phase and amplitude relations, so that an intermediate frequency signal is obtained whose frequency corresponds to the frequency difference between the transmitted and the received signal. The intermediate frequency signals may be evaluated in an electronic evaluation device. For example, they may be digitized using analog/digital converters and then further processed digitally. For example, a frequency spectrum of the intermediate frequency signal is recorded in every channel of the evaluation device during each measurement period. In this frequency spectrum, each located object is indicated by a peak whose frequency range is a function of the distance and the relative velocity of the object in question. By modulating the transmitted frequency using different ramp gradients, it is possible to compute the distance and the relative velocity from the obtained frequency ranges.
The dependency of the amplitude and phase of the signal received on an antenna element on the azimuth of the located object may be illustrated in an antenna diagram for a standard object at a given distance and having a given reflection intensity. By aligning the amplitudes and/or phases obtained for the same object from different antenna elements with the appropriate antenna diagrams, the azimuth of the object in question may be determined in a second evaluation stage.